It is known that large, thin articles are difficult to form by injection moulding. The reason is that the gap between the two parts of the mould is small and the distance that the material has to travel is too long for the pressure applicable by the moulding machine to be available at the far end of the gap from the injection point for driving the plastics to fill the mould. In short, the “flow path thickness ratio” is too long.
Conventionally, thin articles are formed by vacuum or pressure forming where a sheet of plastics material is stretched to conform to the shape of a mould. Such techniques are limited in their application as they cannot produce articles of even wall thickness or articles that have regions of increased or reduced wall thickness. This is because only one surface of the article is being moulded and the thickness at any point is determined exclusively by the thickness of the original sheet and the extent of its deformation.
This problem has been overcome by EP 1360057, which discloses injecting molten plastics material into an open mould forcing the plastics material to fill the mould by closing it at high pressure after injection. This method enables the use of cheaper materials which do not flow as well yet ensures that thin wall sections can be easily made.
It was known prior to EP 1360057 to move part of a mould in order to apply additional compression after having injected a plastics melt into a mould cavity in the conventional manner. This process, which was known as injection compression moulding (ICM) offered advantages of longer flow lengths, thinner walls and a lower level of material stresses. This made and still makes the process suitable for moulding such articles as CDs and DVDs (because of improved internal stresses) and vehicle body and instrument panels (because of improved impact resistance).
The known ICM processes differed from EP 1360057 in the extent of the compression of the plastics melt by the closing of the mould cavity. In the newer technology, the relative displacement of the mould parts is in excess of ten times the final mould thickness and may be as great as two hundred times the final moulding thickness. This was in contrast with known ICM processes, where a corresponding movement of typically twice the final wall thickness was used.
The present invention is concerned with an improvement of the apparatus described in EP 1360057.
According to the present invention, there is provided an injection compression moulding apparatus comprising two mould parts which define a mould cavity and are mounted on two relatively movable platens of an injection moulding press, wherein an actuator is arranged in series with the platen acting on at least one of the mould parts and is controlled in synchronism with the movement of the platens to enable the relative speed of the two mould parts during a mould closing phase of each article moulding cycle to be modified, the speed of relative movement of the mould parts being the vector sum of the speeds of movement and the platens and the actuator.
The present invention is based on the realisation that, during the final stage of closing the mould speed is a more important parameter than pressure in achieving moulded articles of uniform thickness and good surface finish. Though injection moulding presses are capable of applying and maintaining the desired high pressure once the mould has been closed, their speed of movement just before the mould is fully closed, is not necessarily optimum. For thinner sections, the speed may be too low and, for thicker sections, the speed may be too high and uncontrollable.
In the case of a press employing a toggle mechanism, the mechanical advantage of the lever system acting between the hydraulic cylinder and the platen augments the closing pressure but at the same time it slows down the rate of movement of the platen. Rather than attempt to modify the press, the present invention overcomes the problem by placing an actuator in series with at least one of the platens to supplement the relative movement of the platens of the press, the actuator being preferably built into the mould.
The actuator may be a fluid operated piston preferably a hydraulic piston.
In the impact moulding apparatus of EP 1360057 it is necessary to allow one of the mould parts to move towards the platen while injection of plastics material into the mould cavity is taking place. To accommodate both these features in a compact manner, the actuator of a preferred embodiment of the invention is constructed as an annular first piston receiving second piston that acts on said one mould part.
Conveniently, the two pistons are forced apart by means of a force sufficiently weak to allow said one mould part to move towards the associated platen when plastics material is injected into the mould cavity before the latter is fully closed.